The present invention relates to a titanium wire reinforced lead composite electrode structure for use in lead-acid batteries.
Every lead-acid battery has a structure in both the positive and negative electrodes that supports the active material and provides current collection. Typically, this structure is a flat grid, but it could be tubes, baskets, flat plates, or a variety of other configurations. To serve its function the grid must be electrically conductive, have sufficient mechanical strength to support the dense active material, must maintain good electrical contact with the active material and have adequate corrosion resistance in the battery environment.
In most present commercial lead-acid batteries the grid is a lead alloy which is stronger than pure lead, and thus capable of supporting the active mass, typically lead and lead dioxide. Other materials have, of course, been considered. Each of these previously known methods of constructing grids has its drawbacks.
All known alloys of lead have greatly inferior corrosion properties compared to pure lead. Thus the electrode grid structure in the positive electrode corrodes over time, losing contact with the active material which causes the capacity of the battery to decline. In fact, the life-cycle of most lead-acid batteries is limited by this corrosion mechanism. Corrosion in the negative electrode on the other hand is generally not a problem. Also, one or more alloying elements in such lead alloys must be held within composition limits as close as 200 parts per million or less.
In the early 1960's there was some investigation into the use of titanium as the grid material in the positive electrode because of its excellent corrosion properties. For example, British Pat. No. 869,618 to Cotton et al discusses the use of titanium structure for lead-acid positive electrodes, employing a coating of noble metals to which the lead dioxide is subsequently applied. It is interesting to note that Cotton et al specifically teach that lead is not a suitable material to employ between the titanium and the active material.
While titanium has very poor corrosion properties in the negative electrode environment it has excellent corrosion properties in the positive electrode environment and could operate with essentially no corrosion. Additionally, titanium has a much lower density than lead and would decrease the weight of the battery. Interest in titanium waned, however, because of difficulties in getting good contact between the lead dioxide active material and the titanium grid. Despite expensive surface treatments and flash coating of noble metals the lead dioxide did not form a proper bond to the titanium grid.
More recently U.S. Pat. No. 4,282,922 to Hartmann disclosed the use of lead coated alumina fibers to provide a lead matrix composite to be used as the positive electrode structure for lead-acid batteries. The disclosure of Hartmann provides a method of using pure lead with its inherently better corrosion resistance.
The alumina fiber/pure lead composite grid appears to have more promise. The fiber reinforcement appears to impart the needed strength and the corrosion of the pure lead grid has been shown to be quite low in early tests. Experimental work to more fully characterize the corrosion properties is still being performed, however, two major drawbacks appear to make this system unattractive. The cost of this material is currently high, and there is little prospect that the cost can be reduced in the foreseeable future.
The main drawback, however, is the lack of a means to translate such technology into a production environment. In the laboratory, reinforced grids are made by placing a reinforcing fiber into each grid member cavity of a mold and then casting lead around the fibers. Such hand lay-up is very time consuming and would be impossible to do in a production facility. What would be needed would be a preform of the alumina fibers that could be dropped into a mold seconds before casting. The technology for joining ceramic fibers into such a preform does not now exist.